Post by ka9q on Dec 29, 2011 19:49:07 GMT -4

Thanks.

Earth (and Titan, I presume) are the only solar system bodies where parachutes really work all the way to the surface. Mars' atmosphere is so thin (like ours at 100,000') that no practical parachute can provide an acceptably low terminal velocity for a landing, so rockets have to be used at the last moment.

Venus, on the other hand, has a supercritical CO2 atmosphere so dense that you don't even need a parachute to land. The Russian landers did use a parachute in the upper atmosphere, but then cut it away and floated gently down to the surface on a small flat ring around the outside of the lander.

Post by chew on Dec 29, 2011 20:11:58 GMT -4

Venus, on the other hand, has a supercritical CO2 atmosphere so dense that you don't even need a parachute to land. The Russian landers did use a parachute in the upper atmosphere, but then cut it away and floated gently down to the surface on a small flat ring around the outside of the lander.

Yeah, the first time I read that it blew my mind.

"No parachute??? Wow. I guess that makes sense. That is too cool."

"Show us a photo of either the descent or the ascent rockets firing and the exhaust plume.And exactly how does a space craft fly when "The engine is NOT ON"" -Stelios at the DIFGoogle search this site

Post by Bob B. on Dec 30, 2011 10:31:04 GMT -4

Wikipedia, without a reference, states the drogues deployed at 24,000 feet and slowed the capsule down to 125 mph. Assuming the 125 mph speed was obtained at 20,000 feet then the capsule on two drogues would have hit at 83 mph (37 m/s).

That doesn't sound quite right unless the drogues have a higher drag coefficient than the mains. If we assume all other things are equal and we're simply reducing the area of the parachutes, then two drogues at 16.5' diameter have 1/40th the area of three mains at 85' diameter. Therefore, under the drogues the terminal velocity should be 401/2 = 6.32 times greater than under the mains. If the splashdown velocity with all the mains deployed is 8.5 m/s, then under two drogues it should be about 54 m/s, or 120 mph.

Post by ka9q on Dec 30, 2011 11:52:29 GMT -4

The LES designed to make sure that the capsule always ended up in the sea during an abort to specifically prevent the possibility of a hard landing (using the pitch control motor).

Only when the wind wasn't blowing too strongly onto land. Wally Schirra's problems with the ground during Apollo 7 started right at launch when the flight director violated a mission rule that Schirra had insisted on to scrub when the onshore winds were above a certain limit.

Post by ka9q on Dec 30, 2011 11:56:56 GMT -4

You mean the astronauts were laying flat? The capsule was suspended at an angle to cut into the water and reduce deceleration.

You're right, it did hang at an angle just for that reason. But the results still weren't guaranteed. I think Apollo 12 had an especially hard landing when the CM happened to hit a wave at just the wrong angle.

Post by Bob B. on Dec 30, 2011 12:09:52 GMT -4

Might the drogue and main parachutes have different designs? Parachutes are usually not solid, they have vents in the center, or they're made from ribbons with slits between them, etc.

It's very possible the designs are different, though I haven't spent much time studying the issue. I'm pretty sure the mains had a vent in them, at least that's what I recall from memory. Don't know about the drogues.

Plugging in all the know quantities for drag force (mass of capsule), velocity, chute area and air density, and then calculating for the drag coefficient, I get a Cd for the main parachutes of 0.7, which sounds very low to me. I've always heard that parachutes are in the 1 to 1.5 range. The vent may likely be part of the reason for the low Cd. My calculation is also slightly flawed in that I've assumed the maximum cross-section of the parachutes are normal to the air flow. The chutes are actually angled some to the flow, so the area should be a little less than I figured, resulting in a higher Cd.

My calculated impact velocity of 120 mph assumed the drogues have the same Cd as the mains. If the drogue Cd is up in the 1.4 to 1.5 range, then I think chew's 83 mph calculation is spot on. Unfortunately I'm far from being an expert on parachutes.

Post by ka9q on Dec 30, 2011 12:17:43 GMT -4

A 35 mph car wreck will easily kill you if "done right", but all capsule-landing crews are in the most survivable mode - lying flat and fully supported by an energy absorbing system, with no possibility of the violent whiplash that can damage the spinal column or collision with other hard objects. Of course, going fast enough, internal injuries will still be fatal even without these trauma modes.

There actually was at least one collision with a hard object. From the Apollo 12 flight report, page 9-29:

Sea-state conditions were fairly rough, and the landing impact was extremely hard. (Editors note: Later information indicates the command module did not enter the water at the nominal 27.5-degree angle, from which it hangs on the parachute system. Engineering judgement indicates that the command module entered the water at an angle of 20 to 22 degrees, which corresponds to an impact acceleration of about 15g. This off-nominal condition is attributed to a wind-induced swing of the command module while it was on the parachutes and to the existing wave slope at contact.) The 16-mm sequence camera had been placed on its bracket in the right-hand rendezvous window to photograph entry but came loose at impact and contacted the Lunar Module Pilot above the right eye. Later inspection of the spacecraft revealed that portions of the heat shield had been knocked loose during impact.

Post by chew on Dec 30, 2011 14:34:09 GMT -4

You mean the astronauts were laying flat? The capsule was suspended at an angle to cut into the water and reduce deceleration.

You're right, it did hang at an angle just for that reason. But the results still weren't guaranteed. I think Apollo 12 had an especially hard landing when the CM happened to hit a wave at just the wrong angle.

Yeah, it was 12. Bean got beaned (heh heh) on the forehead by a camera that broke loose from it's mount from the violent impact. He needed 6 stitches.

Post by chew on Dec 30, 2011 15:08:59 GMT -4

Might the drogue and main parachutes have different designs? Parachutes are usually not solid, they have vents in the center, or they're made from ribbons with slits between them, etc.

It's very possible the designs are different, though I haven't spent much time studying the issue. I'm pretty sure the mains had a vent in them, at least that's what I recall from memory. Don't know about the drogues.

Plugging in all the know quantities for drag force (mass of capsule), velocity, chute area and air density, and then calculating for the drag coefficient, I get a Cd for the main parachutes of 0.7, which sounds very low to me. I've always heard that parachutes are in the 1 to 1.5 range. The vent may likely be part of the reason for the low Cd. My calculation is also slightly flawed in that I've assumed the maximum cross-section of the parachutes are normal to the air flow. The chutes are actually angled some to the flow, so the area should be a little less than I figured, resulting in a higher Cd.

My calculated impact velocity of 120 mph assumed the drogues have the same Cd as the mains. If the drogue Cd is up in the 1.4 to 1.5 range, then I think chew's 83 mph calculation is spot on. Unfortunately I'm far from being an expert on parachutes.

Found the problem: the mains aren't 85 feet in diameter when in use. They are 85 feet in diameter when spread out on a floor. Their cross-section diameter when in use is about 57 feet. Plugging that into the free fall equation with a Cd of 1.5 results in 8.6 m/s.

"Show us a photo of either the descent or the ascent rockets firing and the exhaust plume.And exactly how does a space craft fly when "The engine is NOT ON"" -Stelios at the DIFGoogle search this site

Post by Bob B. on Dec 30, 2011 15:17:03 GMT -4

Found the problem: the mains aren't 85 feet in diameter when in use. They are 85 feet in diameter when spread out on a floor. Their cross-section diameter when in use is about 57 feet. Plugging that into the free fall equation with a Cd of 1.5 results in 8.6 m/s.